The present invention relates to the conduction and transport of deep-cooled (cryogenic) fluids, such as liquids or gases, and more particularly the invention relates to a conduit system for the transport of low temperature fluids, including at least two concentrically arranged metal tubes spaced by means of a helical spacer, and wherein the annular space between the two metal tubes is evacuated.
U.S. Pat. No. 4,259,990 discloses a conduit system, including an inner tube and an outer tube with a thermally insulating spacer of helical configuration being disposed between the two tubes, the spacer being comprised of a plurality of braided strand elements forming a single non-tubular coreless braid. The particular strand elements as disclosed in this patent are made of a synthetic material such as polyethylene or polytetrafluoroethylene. This kind of configuration is well suited to thermally isolate the interior of the inner tube from the exterior of the outer tube. It was found, however, that after the gap space between the two tubes has been evacuated, the vacuum begins to deteriorate.
It has been discovered that certain residual gases may emerge from the strand elements and establish a gaseous atmosphere between the two tubes. Even though the pressure remains quite low, a certain deterioration of the thermal insulation between the two tubes becomes inevitable. Moreover, it was found that this particular construction is not suitable for the transport of oxygen because if oxygen for some reason or another enters the gap space, sudden inflammation may occur because these synthetic materials of which the braided spacer is made is likewise inflammable. Moreover, it was found that these particular synthetics are not that insensitive to temperature variations, so that particularly, for example, in the case of a temperature increase over a range in excess of 100 degrees centigrade, the spacer will deform, and a uniform distance in radial direction between the outer and the inner tube is no longer insured.